Circuitry using switching elements as resistors is used to provide a variable resistance between two elements. By activating or deactivating switching elements, such as metal-oxide-semiconductor (MOS) transistors, the resistance provided by the MOS transistors is altered. As the size of the MOS transistor increases, the resistance across the active transistor decreases. However, the resistance reduction in relation to the size increase of the MOS transistor diminishes as the MOS transistor size increases.
In a straight binary control arrangement, a plurality of MOS transistors is electrically connected in a parallel arrangement with each MOS transistor having a different size. The straight binary control arrangement is easier to design than other control arrangements and facilitates switching between different resistance values. However, a separation between the different resistance values is not uniform, which in some instances, limits the application of the straight binary control arrangement. The straight binary control arrangement also experiences resistance saturation for large MOS transistor sizes, limiting a resistance tuning range.
In a thermal control arrangement, MOS transistors are electrically connected in parallel, with each MOS transistor having substantially the same size. The thermal control provides greater uniformity between the different resistances than the straight binary control arrangement. In a thermal control arrangement, each of the plurality of MOS transistors has a line electrically connected to a gate of the MOS transistor. As the number of MOS transistors increases, the number of lines in the control bus also increases. For thermal control arrangements having a wide resistance tuning range, the complexity and size of a layout of the thermal control arrangement is greater than the straight binary control arrangement.